Methods and apparatuses for driving audio and ultrasonic signals from the same transducer

ABSTRACT

Driver circuitry is disclosed for driving an electroacoustic transducer to provide an output comprising both ultrasonic and audio signal components. The driver circuitry comprises an adjustment module configured to reduce the level of said ultrasonic component signal in response to an increase in an operational variable indicative of a level of said audio signal component, while also increasing the pulse duration, duty cycle, repetition frequency or frequency span or bandwidth of the ultrasonic component.

FIELD

The field of representative embodiments of this disclosure relates tomethods, apparatuses, or implementations concerning or relating todriving ultrasonic and audio signals from the same transducer withoutintroducing audible artefacts.

BACKGROUND

Recent devices such as smartphones are being provided with ultrasonictransmission and detection capabilities to enable functions such asproximity detection and gesture recognition inter alia.

FIG. 1 illustrates a smartphone 10 comprising a loudspeaker 20, which issupplied with audio signals such as voice or music from audio processingcircuitry 30 and ultrasonic signals from ultrasonic processing circuitry35, and which transmits a combined acoustic output 91. The device mayalso comprise a control or applications processor 40 and a module 45 forreceiving radio frequency signals for communication with a local networkor a public telecommunications network. The smartphone 10 may alsocomprise a microphone 50 to receive reflected waves 92 which maycomprise audio signals such as a user's voice or ultrasonic returnsignals reflected from some nearby object 90. Using the same transducer20 to generate both audio and ultrasonic acoustic signals may savematerial and assembly cost and allow reduced dimensions or allow spacefor extra functions on device 10.

However, when audio and ultrasonic signals are driven from the sametransducer audible distortion may be heard by the user. For instance, asillustrated in FIG. 2, an audio tone at 9 kHz may interact with anultrasonic tone at 22 kHz to give intermodulation products at 22 kHz±9kHz, which will thus provide audible components at 13 kHz, as well as anultrasonic intermodulation component at 31 kHz. Further intermodulationproducts may occur at e.g. 22 kHz−(2×9 kHz)=4 kHz. This interactionbetween the original tones may be caused by non-linearities due to avariety of mechanical, electrical and acoustic effects including forexample excursion limits or non-linearities in transducer parameterssuch as the force factor (Bl) or the membrane compliance Rms.

Such intermodulation products are generally not harmonically related tothe original audio frequency, so are particularly subjectivelyobjectionable. Also in some embodiments the ultrasonic signal may be aswept frequency stimulus such as a chirp or may be modulated infrequency according to some digital or analog data, which may lead tothe intermodulation products also changing in frequency over time andbecoming more noticeable. In many applications the ultrasonic signal maybe in the form of periodic bursts of signal which may generate a morecomplex spectrum and may thus also generate additional intermodulationartifacts.

SUMMARY

According to an aspect of the invention, there is provided drivercircuitry, configurable to provide an output signal comprising anultrasonic component and an audio signal component, the driver circuitrycomprising:

-   -   an ultrasonic signal generation module, responsive to an        operational variable indicative of a level of said audio signal        component, for generating the ultrasonic component with an        amplitude, and a pulse duration, duty cycle, repetition        frequency or frequency span or bandwidth, that are dependent on        the level of said audio signal component, such that a higher        level of said audio signal component leads to a lower amplitude,        and a higher pulse duration, duty cycle, repetition frequency or        frequency span or bandwidth of the ultrasonic component, and a        lower level of said audio signal component leads to a higher        amplitude, and a lower pulse duration, duty cycle, repetition        frequency or frequency span or bandwidth of the ultrasonic        component.

Said level of said audio component may be indicated by an operationalvariable indicating the presence of an audio input signal receivable atan input, or by an operational variable indicating the level of anupstream volume control applied to an audio signal prior to receivingthe audio input signal receivable at an input, or by an operationalvariable indicating the level of a gain to be applied to an audio inputsignal received at an input to provide said audio signal component, orby an operational variable derived from an audio input signal receivedat an input, or by an operational variable derived from anacousto-electrical transducer monitoring an acoustic output of theelectroacoustic transducer.

Said driver circuitry may also comprise: an input for receiving anultrasonic input signal; and said ultrasonic signal generation modulemay be configured to control the level of said ultrasonic componentsignal by applying a controlled gain to said ultrasonic input signal.

Said ultrasonic signal generation module may be configured to generatean ultrasonic source signal and to generate the ultrasonic componentsignal by altering an amplitude of the ultrasonic source signal.

The ultrasonic signal generation module may be configured to generatethe ultrasonic source signal from a stored waveform. The stored waveformmay comprise the waveform of a signal burst.

The ultrasonic signal generation module may be configured to generatethe stored waveform, on re-initialization, and store it for subsequentuse.

The ultrasonic signal generation module may be configured to generatethe stored waveform based on a random bit sequence.

The ultrasonic signal generation module may be configured to generatethe ultrasonic source signal from a waveform received from an externalsource.

Said ultrasonic signal generation module may be configured to generatethe ultrasonic component with a required amplitude, and with a requiredpulse duration, duty cycle, repetition frequency and frequency span orbandwidth.

Said ultrasonic signal generation module may be configured to generatethe ultrasonic component signal by selecting from a plurality of storedwaveforms corresponding to a set of respective required characteristics.

Said ultrasonic signal generation module may comprise a first waveformgenerator and a second waveform generator, and the ultrasonic signalgeneration module may be configured to generate the ultrasonic componentas an output either of the first waveform generator or of the secondwaveform generator, based on a value of said operational variable.

The first waveform generator may be configured for generating a waveformof a first type and the second waveform generator may be configured forgenerating a waveform of a second type, wherein the second type ofwaveform is simpler than the first type, and wherein the ultrasonicsignal generation module may be configured to generate the ultrasoniccomponent as an output of the first waveform generator when the value ofsaid operational variable indicates that the level of said audio signalcomponent is above a threshold level.

The waveform of the first type may be a wideband noise-like waveform.

The first waveform generator may comprise: a digital noise generator,for generating a noise sequence of values; a band-pass filter, forreceiving the noise sequence and limiting the bandwidth thereof to anultrasonic range; and a window for selecting a set of samples outputfrom the band-pass filter, corresponding to a required pulse duration.

The digital noise generator may comprise a pseudo-random bit sequencegenerator, for generating a random sequence of values with a flatwideband frequency spectrum.

The required pulse duration, and an amplitude of the waveform, may beadjustable.

The bandwidth of the waveform may be adjustable.

The waveform of the second type may be a chirp.

The waveform of the second type may be a single tone.

The ultrasonic component may comprise a plurality of bursts, and atleast the amplitude of the ultrasonic component may be kept constantduring each burst, and be variable between bursts.

The driver circuitry may further comprise: a first input for receivingan audio source signal; and an output for providing electrical signalcomponents to be applied to said transducer.

Said operational variable of the audio component electrical signal maybe indicative of the amplitude of an envelope of the audio componentelectrical signal. Said envelope may be derived according to an attacktime, a decay time or a hold time.

The ultrasonic signal generation module may comprise a variable gainmodule configured to apply a variable gain to an ultrasonic sourcesignal according to a gain control signal based on the parameter of theaudio source signal.

The variable gain module may comprise a digital multiplier for applyingthe variable gain to the ultrasonic source signal.

The variable gain module may comprise a digital-to-analogue converteremploying a reference voltage which is adjusted for applying thevariable gain to the ultrasonic source signal.

The driver circuitry may comprise a combining module for combining theultrasonic and audio signal component electrical signals beforeapplication to the transducer.

According to an aspect of the invention, there is provided drivercircuitry, comprising:

-   -   a waveform generator, wherein the waveform generator comprises:    -   a digital noise generator, for generating a noise sequence of        values;    -   a band-pass filter, for receiving the noise sequence and        limiting the bandwidth thereof to an ultrasonic range; and    -   a window for selecting a set of samples output from the        band-pass filter, corresponding to a required pulse duration.

The digital noise generator may comprise a pseudo-random bit sequencegenerator, for generating a random sequence of values with a flatwideband frequency spectrum.

The band-pass filter may limit the bandwidth of the pseudo-random bitsequence to an ultrasonic frequency range.

The window may be a Gaussian time window.

The driver circuitry may further comprise a memory for storing theselected set of samples. The driver circuitry may then further comprisean output for repeatedly generating pulses from the stored selected setof samples.

The repetition rate of said pulses may be adjustable, the bandwidth ofsaid pulses may be adjustable, the duty cycle of said pulses may beadjustable, and/or the duration of said pulses may be adjustable.

The driver circuitry may further comprise a gain element, for alteringsample values of the selected set of samples, wherein the memory isconnected to the gain element for storing the selected set of sampleswith altered sample values.

According to an aspect of the invention, there is provided drivercircuitry for driving an electroacoustic transducer to concurrentlyprovide ultrasonic and audio acoustic signal components, comprising:

-   -   a first input for receiving an audio source signal;    -   an output for providing electrical signal components to be        applied to said transducer, said electrical signal components        comprising an audio component electrical signal derived from        said audio source signal and an ultrasonic component electrical        signal;    -   parameter estimation circuitry configured to provide an        operational variable based on the voltage and current of the        electroacoustic transducer and a plant model of the        electroacoustic transducer; and    -   an ultrasonic generation module for providing said ultrasonic        component electrical signal with a level dependent on said        operational variable.

A repetition rate of pulses of said ultrasonic component may bedependent on said operational variable, a bandwidth of pulses of saidultrasonic component may be dependent on said operational variable, aduty cycle of pulses of said ultrasonic component is dependent on saidoperational variable, and/or a duration of pulses of said ultrasoniccomponent may be dependent on said operational variable.

Said operational variable may comprise one or more of: the displacementof a membrane, velocity of a membrane, acceleration of a membrane,temperature of a voice coil, or estimated SPL of speaker.

According to an aspect of the invention, there is provided an electronicdevice comprising such driver circuitry.

According to an aspect of the invention, there is provided a method ofgenerating a transducer drive signal comprising an ultrasonic componentand an audio signal component, the method comprising:

-   -   determining an operational variable indicative of a level of the        audio signal component; and    -   generating the ultrasonic component with an amplitude, and a        pulse duration, duty cycle, repetition frequency or frequency        span or bandwidth, that are dependent on the level of said audio        signal component, such that a higher level of said audio signal        component leads to a lower amplitude, and a higher pulse        duration, duty cycle, repetition frequency or frequency span or        bandwidth of the ultrasonic component, and a lower level of said        audio signal component leads to a higher amplitude, and a lower        pulse duration, duty cycle, repetition frequency or frequency        span or bandwidth of the ultrasonic component.

According to an aspect of the invention, there is provided a computerprogram, comprising computer-readable code for causing a processor toperform the method.

According to an aspect of the invention, there is provided an electronicdevice, comprising a processor and a memory, the memory storing thecomputer program, for causing the processor to perform the method.

According to an aspect of the invention, there is provided drivercircuitry for driving an electroacoustic transducer to provide an outputcomprising both ultrasonic and audio signal components, said drivercircuitry comprising:

an adjustment module configured to reduce the level of said ultrasoniccomponent signal in response to an increase in an operational variableindicative of a level of said audio signal component.

Said increase in level of said audio component is indicated by anoperational variable derived from an audio input signal received at aninput.

Said increase in level of said audio component is indicated by anoperational variable derived from an acousto-electrical transducermonitoring an acoustic output of the electroacoustic transducer.

Said driver circuitry may also comprise:

an input for receiving an ultrasonic input signal; and

said adjustment module may be configured to reduce the level of saidultrasonic component signal by applying a controlled gain to saidultrasonic input signal.

Said adjustment module may be configured to generate an ultrasonicsource signal and to reduce the level of said ultrasonic componentsignal by generating a reduced amplitude ultrasonic source signal.

Said adjustment module may be configured to increase the pulse duration,repetition frequency or frequency span of said ultrasonic componentsignal when reducing the amplitude of said ultrasonic component signal.

According to an aspect of the invention, there is provided drivercircuitry for driving an electroacoustic transducer to provide an outputcomprising simultaneous ultrasonic and audio signal components, saiddriver circuitry comprising:

an adjustment module for reducing the level of said ultrasonic componentsignal in response to a comparison of an audio input signal and anestimated acoustic audio output.

Said estimated acoustic audio output may be estimated based on thevoltage and current of the electroacoustic transducer and a plant modelof the electroacoustic transducer.

Said estimated acoustic audio output may be estimated based on theelectrical output of an acousto-electrical transducer.

According to an aspect of the invention, there is provided drivercircuitry for driving an electroacoustic transducer to concurrentlyprovide ultrasonic and audio acoustic signal components, comprising:

-   -   a first input for receiving an audio source signal;    -   an output for providing electrical signal components to be        applied to said transducer, said electrical signal components        comprising an audio component electrical signal derived from        said audio source signal and an ultrasonic component electrical        signal; and    -   an adjustment module for providing said ultrasonic component        electrical signal configured to adjust a level of said        ultrasonic component electrical signal based on an operational        variable indicative of the level of the audio component        electrical signal,    -   wherein said adjustment module reduces the amplitude of said        adjusted ultrasonic component electrical signal when a higher        level of the audio component electrical signal is indicated to        be present.

Said operational variable of the audio component electrical signal maybe indicative of the amplitude of an envelope of the audio componentelectrical signal. Said envelope may be derived according to an attacktime, a decay time or a hold time.

The adjustment module may comprise a variable gain module configured toapply a variable gain to the ultrasonic source signal according to again control signal based on the parameter of the audio source signal.

The variable gain module may comprise a digital multiplier for applyingthe variable gain to the ultrasonic source signal.

The variable gain module may comprise a digital-to-analogue converteremploying a reference voltage which is adjusted for applying thevariable gain to the ultrasonic source signal.

The driver circuitry may comprise a combining module for combining theultrasonic and audio signal component electrical signals beforeapplication to the transducer.

Said adjustment module may be configured to increase the pulse duration,repetition frequency or frequency span of said ultrasonic componentsignal when reducing the amplitude of said ultrasonic component signal.

According to an aspect of the invention, there is provided drivercircuitry for driving an electroacoustic transducer to concurrentlyprovide ultrasonic and audio acoustic signal components, comprising:

-   -   a first input for receiving an audio source signal;    -   an output for providing electrical signal components to be        applied to said transducer, said electrical signal components        comprising an audio component electrical signal derived from        said audio source signal and an ultrasonic component electrical        signal;    -   parameter estimation circuitry configured to provide an        operational variable indicative of the acoustic signal output of        the transducer; and an adjustment module for providing said        ultrasonic component electrical signal configured to adjust a        level of said ultrasonic component electrical signal based on        said operational variable.

Said operational variable may be derived based on the electrical outputof an acousto-electrical transducer.

Said operational variable may be derived based on the voltage andcurrent of the electroacoustic transducer and a plant model of theelectroacoustic transducer.

Said operational variable may comprise one or more of: the displacementof a membrane, velocity of a membrane, acceleration of membranetemperature of voice coil estimated SPL of speaker.

The adjustment of the level of said ultrasonic component electricalsignal may be based on a comparison signal derived from the audio inputsignal and said operational variable.

The adjustment of the level of the ultrasonic component electricalsignal may be based on an adaptive iterative method.

The adjustment module may also be configured to apply a non-linearity tosaid ultrasonic component electrical signal.

According to an aspect of the invention, there is provided drivercircuitry for generating an ultrasound signal to be applied to aloudspeaker, wherein the ultrasound signal is controlled based on anoperational variable indicative of a property of an audio signalconcurrently applied to the loudspeaker.

An amplitude, frequency spectrum or time domain feature of theultrasound signal may be controlled based on said operational variable.

The operational variable may be based on an input audio signal fromwhich said audio signal is derived.

The driver circuitry may further comprise circuitry for detecting soundgenerated by the loudspeaker, wherein the ultrasound signal iscontrolled based on the detected sound generated by the loudspeaker.

The driver circuitry may further comprise circuitry for detecting one ormore operational parameter of the loudspeaker, wherein the ultrasoundsignal is controlled based on the detected one or more operationalparameter of the loudspeaker.

According to another aspect of the invention, there is provided anelectronic device comprising driver circuitry according to any precedingaspect.

The electronic device may be at least one of: a portable device; abattery powered device; a communications device; a computing device; amobile telephone; a laptop, notebook or tablet computer; a personalmedia player; a gaming device; and a wearable device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a smartphone.

FIG. 2 illustrates audio and ultrasonic tones.

FIG. 3 illustrates an electroacoustic transducer and associated drivercircuitry.

FIG. 4 illustrates a further electroacoustic transducer and associateddriver circuitry.

FIG. 5 shows an example of waveforms in the circuitry of FIG. 4.

FIG. 6 illustrates an effect of driving an audio signal and anultrasonic signal concurrently.

FIG. 7 illustrates a relationship between ultrasonic power and audiopower in the circuitry of FIG. 4.

FIG. 8 illustrates an ultrasonic generation module.

FIG. 9 illustrates a further ultrasonic generation module.

FIG. 10 illustrates a further electroacoustic transducer and associateddriver circuitry.

FIG. 11 illustrates a further electroacoustic transducer and associateddriver circuitry.

FIG. 12 illustrates a further electroacoustic transducer and associateddriver circuitry.

FIG. 13 illustrates a further electroacoustic transducer and associateddriver circuitry.

FIG. 14 illustrates a further electroacoustic transducer and associateddriver circuitry.

FIG. 15, views (a), (b), (c), and (d) illustrate circuitry for drivingsignals onto an electroacoustic transducer.

DETAILED DESCRIPTION OF EMBODIMENTS

The description below sets forth example embodiments according to thisdisclosure. Further example embodiments and implementations will beapparent to those having ordinary skill in the art. Further, thosehaving ordinary skill in the art will recognize that various equivalenttechniques may be applied in lieu of, or in conjunction with, theembodiments discussed below, and all such equivalents should be deemedas being encompassed by the present disclosure.

The embodiments relate to methods or apparatus which eliminate orsignificantly reduce audible distortion caused by electrical, mechanicalor acoustic non-linearities in an electro-acoustic transducer whentransmitting both an audio signal and an ultrasonic signal, by modifyingthe ultrasonic signal in response to the operational variables relatedto the audio signal at the input and/or the output.

For the purposes of this document, audio signals may be considered asthose signals audible by a human, conventionally taken as up to 20 kHz,although some audio signals may be limited in bandwidth bypre-processing or channel limitations to extend to a lower maximumfrequency, say 8 kHz or 3.4 kHz for example. We will refer to either anultrasonic (>20 kHz) or near ultrasonic (e.g. in the upper half of theaudio band) signal as an ultrasonic signal. “Acoustic” covers thepressure waves in a medium due to sound in both audio and higherultrasonic bands, as contrasted to electrical signals which mayrepresent such sounds.

FIG. 3 illustrates an embodiment. An output electro-acoustic transducerfor example a loudspeaker 20 receives a signal with audio bandcomponents Ca based on an audio source signal Va, and receivesultrasonic components Cu from an ultrasonic generation module 101. Inthis illustrated embodiment, an ultrasonic source signal Vu is adjustedor modified by the ultrasonic generation module 101 to provide Cu, andthe adjustment is dependent on the input audio signal Va. The signalcomponents Ca, Cu are combined in a combination block here illustratedas an adder 102.

In some embodiments the ultrasonic source signal may be generated withinthe ultrasonic generation module 101 rather than received from someother source. In some embodiments the required ultrasonic signal may begenerated directly within the ultrasonic generation module 101, ratherthan an ultrasonic source signal being generated and then adjusted ormodified.

There may be other signal processing (not illustrated) in the signalpath between an audio source signal input where Va is received and theblock 102 where the signal components are combined.

There may be some amplifier or digital-to-analogue circuitry upstream ofand in series with the actual physical loudspeaker or transducer 20, notillustrated for simplicity.

FIG. 4 illustrates an embodiment in more detail. In this embodiment ofthe ultrasonic generation module 101, the audio source signal input iscoupled to an audio parameter extraction module 110 coupled in turn to acontrol module 120 coupled in turn to a signal adjustment module 130.Audio parameter extraction module 110, for example an envelope detector,provides an audio signal parameter Ya based on the audio source signal.This parameter may be indicative of the level of the audio signal to berendered as an acoustic signal by the transducer 20. Audio signalparameter Ya is received by control module 120, for instance a digitallook-up table, which receives Ya and generates an ultrasonic gain valueGus. Ultrasonic gain value Gus is applied to control the signaladjustment module 130, comprising a variable gain stage 132, forinstance comprising a digital multiplier or analog programmable gainamplifier, to apply a corresponding gain to the ultrasonic source signalVu to generate the ultrasonic component signal Cu.

Thus, the ultrasonic signal may be modified by applying a gain to theultrasonic input signal which is varied in response to a parameter oroperational variable indicative of the level of the audio signal beingplayed. In some embodiments, the ultrasonic component signal Cu may begenerated directly within the adjustment module 101 with an amplitudecorresponding to the gain value Gus, rather than an ultrasonic sourcesignal being generated and then adjusted or modified by an explicitvariable gain stage.

In embodiments control module 120 may also generate other controlsignals Cus which may alter the output ultrasonic component signal Cuforwarded by signal adjustment module 130 to combination block 102, asdescribed below.

FIG. 5 illustrates an example input audio source waveform Va, theresulting gain signal Gus, and resulting ultrasonic component signal Cuon upper, middle, and lower traces respectively. In this example, theaudio signal parameter Ya is a short-term envelope of the audio sourcesignal Va and the gain control module generates a gain that continuouslyand instantaneously follows the envelope signal and is immediatelyapplied to the ultrasonic source signal.

Thus, during the time span illustrated the envelope Ya of the audiosource signal Va in the upper trace decreases then increases. Inresponse the gain control signal Gus in the middle trace increases thendecreases, so that the amplitude of the ultrasonic component signal inthe lower trace increases when the audio signal envelope decreases.

In embodiments, the ultrasonic signal may be of the form of bursts ofsignal. The gain applied to the signal may vary within each burst asillustrated in FIG. 5 or each burst may be subjected to a respectivegain adjustment, constant during the duration of each burst.

In embodiments, the envelope detector may detect the amplitude of theinput signal with an attack time, a decay time or a hold time. Theattack time may be shorter than the decay time so as to allow a rapidresponse to any sudden increase in the audio signal. A hold time ordecay time may be comparable to audio signal periods so that theenvelope tracks the amplitude of the audio waveform in detail.Alternatively, a decay time or hold time may be deliberately longer thanan audio signal period, for example longer than 10 ms, so that only thegeneral long-term level of the audio signal is monitored, to avoid rapidmodulation of the ultrasonic signal, which may itself generate sidebandsin the ultrasonic component signal.

In some embodiments the gain applied to the ultrasonic component signalmay be based on a volume control signal which may also control a gainapplied to the audio source signal within the driver circuitry or may beupstream of the audio source input. The volume signal may be indicativeof the level of the audio component signal corresponding to a full-scaleor maximum signal anticipated to be provided as an original audio sourcesignal.

In some embodiments, the volume control signal may include a settingwhere the gain applied to the audio source signal is zero, or maycontain an indication that the audio signal is silence or that the userdoes not currently wish for any audio signal to be played back. Theultrasonic signal component may then be a maximum amplitude.

In some embodiments the gain applied to the ultrasonic signal may beadjusted in a continuous or quasi-continuous fashion, by a numericalcalculation using a predefined gain law or by a look-up table embodyinga pre-determined gain characteristic. In other embodiments, the gain tobe applied may be limited to a small number of discrete levels, say twoor less than 8.

The gain law to be applied for a particular transducer may be derived bycomputer modelling of the non-linearities, or empirically frommeasurement FIG. 6 illustrates the total power of audio bandintermodulation products versus power of the ultrasonic components inthe presence of various amounts of concurrently applied audio signal ofa particular frequency, say 9 kHz. In the absence of any audiocomponent, an ultrasonic signal component power of Pmax may be allowedto be delivered, limited by other effects, for example the maximum powerof the amplifier, or some specified acoustic safety limit or maximumpower that the speaker can handle without over-temperature issues.

If a −40 dB audio signal is applied, it may be found from measurementthat an ultrasonic stimulus of power P1 will give rise to a total audioband distortion power equal to some specified limit Dmax. Audio signalsof power −20 dB or 0 dB may give similar audio band distortion power forultrasonic powers of P2 and P3 respectively. Thus a plot of maximumultrasonic power versus audio band component power may be derived asillustrated in FIG. 7, from which the required attenuation to be appliedto a source signal of power Pmax may be derived and interpolated.

As discussed above, rather than actually generating an ultrasonic sourcesignal Vu and explicitly adjusting a gain applied to generate anultrasonic component signal Cu, in some embodiments the ultrasoniccomponent signal Cu may be directly generated with an appropriateamplitude. In some embodiments, the ultrasonic component signal Cu maybe generated from a stored waveform, for instance comprising thewaveform of a complete burst of signal, to which gain is applied. Insome embodiments, the ultrasonic component signal Cu may be generated byselecting from a plurality of stored waveforms corresponding to a set ofrespective amplitudes or effective gain value settings.

By reducing the amplitude of the ultrasonic components of the outputsignal provided to the transducer when large amplitude audio componentsrequire to be reproduced, the combined signal may be maintained within acertain maximum amplitude, and thus avoid clipping of the combinedsignal in the output amplifier and resulting audio artifacts. From auser perspective, the automatic adjustment of the ultrasonic componentamplitude also allows louder music playback than would be the case if afixed larger margin were required to accommodate a fixed amplitudeultrasonic component.

However, while it is necessary for the combined signal not to exceed acertain maximum amplitude, i.e. to lie within a certain range, forexample the full-scale range of a digital signal or the supply voltageof an analog amplifier, this is not a sufficient condition to avoidsignificant intermodulation components falling into the audio band. Highamplitude intermodulation products may be found in the audio band evenwhen the combined signal is well below a clipping level.

The issue may not be speaker excursion non-linearities, which aregenerally only significant for frequencies below the resonance of aloudspeaker, and usually require mainly limiting of the applied lowfrequency components of an audio signal. Even for a micro-speaker, theresonance frequency is generally under 1 kHz, yet these effects areapparent for frequencies around a decade above that, where speakerexcursion is small.

However, reducing the level of the ultrasonic component may result in aloss in signal-to-noise ratio (SNR) of any resulting signal, for examplea reflected signal, received by an acousto-electrical transducer, e.g. amicrophone, in the host device, e.g. a smartphone. This may result inimpaired parametric performance of any application relying on receivingand interpreting an ultrasonic signal, or even prevent functionality.

It is known to provide and process ultrasonic signals in the form ofsignals spread over a frequency band rather than single tone bursts. Forexample a chirp waveform may be used comprising a tone which is swept infrequency from a lower frequency to a higher frequency or vice versa orswept across a similar frequency range in both directions. The pulsecompression gain of a chirp may be defined as G_(PULSE)=T ΔF where T isthe duration of the pulse and ΔF is the bandwidth of the chirp, that is,the frequency span over which the tone is swept during the chirp. Theeffective gain may also be increased by increasing the repetition rateFrep of the pulse or chirp. The effective gain may also be increased byincreasing the duty cycle of the pulse or chirp, given that the dutycycle is the product of the pulse duration and the repetition rate.

Thus parameters of the chirp waveform may be varied to increase thepulse compression gain of a chirp stimulus and hence improve the SNR. Inembodiments, the bandwidth or pulse duration, duty cycle, or repetitionrate or other parameter of the ultrasonic component Cu of thetransmitted pulse may be varied in accordance with a parameter, e.g.level, of the audio component signal level. To regain the SNR lost bylowering the signal level, the duration of the pulse, or the duty cycleof a sequence of pulses, or the bandwidth or repetition rate of thechirp may be increased as the ultrasonic component signal level drops.

In some embodiments ultrasonic signals other than chirp signals mayadvantageously be employed. FIG. 8 illustrates an ultrasonic signalgenerator 142 for generating wideband noise-like ultrasonic signals.Block 132 is a digital noise generator, for generating a noise sequenceof values. In some embodiments, the digital noise generator is a noisegenerator (that is also used for different purposes) that takes a(possibly amplified) analog noise and digitises it to provide the noisesource. In other examples, for example as illustrated in FIG. 8, thedigital noise generator is a shift-register based pseudo-random sequencegenerator 132 as is known, which generates a random sequence ofmulti-bit values, which have a flat wideband frequency spectrum.

The bit sequence generator may be controllable, so that the range of thesample values, and hence the amplitude of the resultant pulse, iscontrollable. This bit sequence is passed through a band-pass filter 133to limit the bandwidth to an ultrasonic range, for instance 20 kHz to 30kHz. The band-pass filter 133 may be controllable, so that the bandwidthof the waveform is controllable. A window 134 selects an appropriate setof samples, corresponding to the duration of each ultrasonic pulse to betransmitted, for example 1024 samples at a 96 kHz sample rate for a 10.7ms pulse width, possibly transmitted at an 80% duty cycle to give a 13.3ms pulse repetition frequency. The window function 134 may becontrollable, so that the pulse duration is controllable. This windowmay preferably be a Gaussian window or some similar smoothing window toprevent signal discontinuities at the start or end of each individualtransmitted ultrasonic pulse.

Thus rather than a single tone, or even a chirp waveform sweepingthrough frequencies, the generated ultrasonic pulse will have a flatspectrum extending over a wide frequency range. Thus if any down-mixingto audio band does occur, it will be spread over the audio band asapparent audio band noise, rather than fixed or sweeping audio tones.

In some embodiments each transmitted ultrasonic pulse may beindividually generated when required. However in some applications suchas motion or gesture detection it may be advantageous for the sameultrasonic waveform to be transmitted every time, so that consecutiveresponses may be subtracted and only the difference signal analysed, toremove fixed echoes and select only reflections that change pulse bypulse, indicating relative movement of the reflecting object. Thus, therandom sequence may be generated only once and the correspondingultrasonic pulse waveform generated only once and stored in suitablememory circuitry 136.

The random sequence or even the ultrasonic waveform may be generatedduring manufacture of the driver circuitry or of a host devicecomprising it. Advantageously however, the random sequence may begenerated infrequently, say on re-initialization of the ultrasonicsignal generator for example on power-up or re-boot of a host device orre-start or re-enabling of the ultrasonic processing circuitry. In thisway the ultrasonic pulse generated by the host device will be randomlydistinct from that generated by other local host devices, reducing thechance of one device erroneously responding to pulses transmitted byanother nearby device.

The amplitude of the generated or stored ultrasonic pulse waveform maybe gain adjusted according to a gain value as described above.Alternatively, a set of waveforms generated by scaling an originalwaveform or by generating a set of appropriate amplitude waveforms asillustrated by Gain block 135, may be stored, and selected later asrequired.

In some instances, it may be advantageous for the waveforms used whenthere is no audio signal (or only a signal of a small amplitude) to bereplayed, to be different from the waveforms used when an audio signalis to be played. For instance, a wideband noise-like waveform as abovewith a single fixed amplitude value, or an amplitude selected from arange of values, may be transmitted in the presence of an audio signal.However, a conventional chirp or even single tone may be adequate whenthere is no audio present and thus no chance of intermodulation of theultrasonic signal with the audio signal. The increased amplitude whenthere is no audio present may give enough signal-to-noise benefit toallow a low, say 5%, duty cycle rather than 80% for the output driveramplifier and for receiver circuitry. This may significantly helpaverage power consumption, and the simpler signal may also allow simplerlower-power processing of the received ultrasonic waveform.

Thus, as illustrated in FIG. 9, the driver circuitry may comprise afirst ultrasonic pulse waveform generator 142 which may generate awideband noise-like waveform as described with reference to FIG. 8, anda second ultrasonic pulse waveform generator 144 which may generate adifferent (maybe simpler) ultrasonic pulse waveform such as a simpletone burst or chirp. The second waveform generator 144 may produceultrasonic signal bursts at a lower duty cycle than the first, allowinga downstream amplifier to also operate at a lower duty cycle.

A control signal Gus controls whether to use the signal from the firstgenerator or the second generator. In some embodiments control signalGus may comprise information directly or indirectly conveyinginformation about the presence of an input signal to be replayed. Forinstance a maximum value of a multi-bit gain control signal may indicateno signal or zero signal amplitude, or alternatively a dedicated controlbit may convey this information, as a result of which the second signalgenerator may be selected.

In some embodiments, as illustrated, different stored waveforms may beselected from the first generator according to the control signal Gus,corresponding to different amplitudes or effective applied gains. Insome embodiments the selected signal may be subject to gain as appliedby illustrated optional gain block 144.

This description assumes that both pulse waveform generators are runningcontinuously, with the control signal Gus being used to select theoutput from one of the two waveform generators. In other embodiments,the control signal Gus may be used to enable and disable the waveformgenerators, so that only one selected waveform generator is running atany one time.

FIG. 9 shows two separate pulse waveform generators. However, inpractice, each pulse waveform generator may be implemented at least inpart by suitable stored program code running on an appropriateprocessor. In that case, the two pulse waveform generators may beimplemented by two components of program code running on one processor.Further, in that case, the control signal Gus may be used to selectwhich of the two components of the program code is running on theprocessor, and therefore which one selected waveform generator isoperating, at any one time.

In any of these embodiments, as described with reference to FIG. 9, theultrasonic signal generation module may comprise a first waveformgenerator and a second waveform generator, and the ultrasonic signalgeneration module is configured to generate the ultrasonic component asan output of the first waveform generator when the value of theoperational variable indicates that the level of the audio signalcomponent is above a threshold level, and as an output of the secondwaveform generator when the value of the operational variable indicatesthat the level of the audio signal component is below the thresholdlevel. More specifically, the first waveform generator may be configuredfor generating a waveform of a first type and the second waveformgenerator may be configured for generating a waveform of a second type,wherein the second type of waveform is simpler than the first type.Thus, the first type of waveform may be the wideband noise-like waveformas described with reference to FIG. 8, and the second type of waveformmay be a simple tone burst or chirp. The second waveform generator mayproduce ultrasonic signal bursts with a higher amplitude, and with alower duty cycle than the first waveform generator.

FIG. 10 illustrates an embodiment where a control block 120 a receivesan envelope signal Ya from an envelope detector 111 in audio parameterextraction module 110 a and generates therefrom not only a gain controlsignal Gus but also control signals defining appropriately adjustedchirp parameters comprising one or more of the lower frequency fL, upperfrequency fH, pulse duration TP, pulse duty cycle, or repetitionfrequency Frep of the chirp waveform. In some embodiments the gainsignal may modulate the amplitude of the chirp waveform as initiallygenerated rather than first generating a normalized waveform and thenscaling it, but such operation is equivalent to notionally generating aunity amplitude chirp signal and then scaling it to the appropriateamplitude.

In the case of the spectrum of the audio signal Ca being very wide, theultrasonic signal can be spread over a wider bandwidth and hidden belowthe acoustic noise and hence regain the SNR.

In some embodiments the chirp waveform (or a wideband noise-like signal)may be allowed to be extended into the audio band, e.g. below 20 kHz, ifit is judged that there is enough spectral energy in the audio signal inthe relevant audio frequency bands to be able to psycho-acousticallymask the ultrasonic components. At least some of the spectrum of theultrasonic chirp is then hidden in the audio signal, but can still berecovered by the processing gain of matched filtering. Thus in thepresence of large enough audio signal the frequency range of the chirpmay be extended further than otherwise possible, recovering some of theloss of SNR that would otherwise occur due to attenuation of theultrasonic signal in the presence of such a large audio signal. Thus asillustrated in FIG. 10, embodiments may comprise a spectral analysisblock 112 which generates data Yas used by the control block 120 a toallow extension of the permissible lower limit of the lower chirpfrequency when possible in view of some spectral parameters of the audiosignal components.

As remarked above, the parameter extraction module 110 a and the controlmodule 120 a may apply fast attack times, slow decay times, or holdtimes to the parameters or control signals to avoid excessivemodulations of the ultrasonic signal gain. There may be further delayadded to any signal to change the chirp amplitude or other parametersmid-chirp, i.e. such parameters may be allowed to change only in betweensuccessive chirp pulses. However, a delay in say reducing the chirpamplitude may result in some audible distortion. Thus, the chirpgeneration may be allowed to be interrupted or rapidly reduced inembodiments where this transient distortion is judged more importantthan the ultrasonic application.

A host device may comprise a transducer to capture received ultrasonicsignals, for example reflected from some nearby object. The device maythus incorporate signal processing to process the received chirpwaveform according to the expected frequency range and pulse duration.

FIG. 11 illustrates an embodiment in which the host device comprises amicrophone 50 whose output is coupled to ultrasonic processing module351. Ultrasonic processing module 351 is coupled to control module 120 ato receive control parameters. In use the acoustic output 91 fromspeaker 20 may be reflected from some object 90 to produce reflectedwaves 92 which are picked up by microphone 50. The electrical outputfrom microphone 50 is then processed by module 351.

The acoustic output 91 may comprise an acoustic ultrasonic chirpcorresponding to ultrasonic component signal Cu generated by theAdjusted Chirp Generator 130 a according to one or more of parametersGus, fL, fH, TP or Frep from the control module 120 a as derived fromaudio signal parameters Ya and/or Yas derived from the audio signal Vaby parameter extraction module 110 a. Thus the ultrasonic signalprocessing in module 351 is performed taking account of theaudio-signal-dependent parameters Gus, fL, fH, TP or Frep of theultrasonic component of the acoustic output.

Other methods and circuitry may be used to provide ultrasonic pulseswith energy smeared over a bandwidth. Embodiments may use such signalformats for transmitted and received ultrasonic pulses with similarlyvariable signal generation parameters to provide variable pulseconversion gain in a similar way to chirp waveforms, for instance thelength or repetition rate of a Barker Code modulated waveform.

Embodiments described above operate in a “feed-forward” fashion in whichthe ultrasonic signal adjustment, e.g. gain, has been based on aparameter indicative of the electrical audio component signal. Furtherembodiments may adjust the ultrasonic component signal based onparameters of the output transducer signal derived for example from thevoltage and current of the transducer or from acoustic signals monitoredby a microphone, e.g. on the host device or close to the outputtransducer. Yet further embodiments may operate where the ultrasoniccomponent signal is adjusted using a combination of the feed-forward andfeed-back methods.

FIG. 12 illustrates an embodiment in which an adaptive scheme is appliedto the ultrasonic path in order to modify the ultrasonic signal in sucha way that the down conversion terms are significantly reduced.

In this embodiment, a microphone 50 a is provided adjacent to thespeaker 20 to detect and reproduce the acoustic output signal x (ratherthan being positioned primarily to pick up an anticipated reflectedsignal). The electrical output x** from microphone 50 a may beband-limited in a low-pass filter 151 to provide an estimated acousticaudio signal x* which has a similar bandwidth as the input audio sourcesignal Va. The filtered acoustic signal x* is then compared to Va bycomparison module 150 here illustrated simply as a subtractor.

In a simple embodiment, the signal adjustment module 130 is a gainelement. The comparison signal E generated by the comparison module 150is then processed in a control block 120 b to provide a gain controlsignal Gus to be applied by the signal adjustment module 130 toattenuate the ultrasonic component signal until the audio band error isbelow a specified limit.

In a more complex embodiment the signal adjustment module 130 mayrepresent an adjustable non-linearity, which is adapted, for exampleusing an adaptive iterative method. For example the non-linearity may bemodified by a polynomial p(x). The m-th polynomial term p_(m) is updatedperiodically, for example at every pulse or chirp by

p _(m) →p _(m) +μ·εx ^(m)

where μ is a convergence factor.

In some embodiments filtering may be applied to the error signal E.

As above, control block 120 b may also provide other control signals Custo the signal adjustment module 130 to adjust other parameters of theultrasonic component signal Cu, for example the duration or duty cycleof ultrasonic signal bursts.

FIG. 13 illustrates an embodiment in which an adaptive scheme is appliedto the ultrasonic path in order to modify the ultrasonic signal in sucha way that the down conversion terms are minimized.

In this embodiment, the speaker current Isp and speaker voltage Vsp areinput to a speaker model module 410 in which a speaker plant modelderives an estimated acoustic signal x**, i.e., an estimate of the soundproduced by the speaker. (This plant model may also be used for speakerexcursion protection or speaker linearization purposes.) The estimatedacoustic signal x** may be band-limited by a low-pass filter 151 to asimilar bandwidth as the input audio source signal Va. The filteredacoustic signal x* is then compared to Va by comparison module 150 hereillustrated simply as a subtractor.

In some embodiments the voltage signal Vsp is derived directly from thecombined signal that is generated in order to be applied to the speaker20, possibly a digital signal, rather than taking the analog voltageimposed on the speaker and converting it to digital.

In a simple embodiment scheme the signal adjustment module 130 is a gainelement. The comparison signal E generated by the comparison module 150is then processed in a control block 120 b to provide a gain controlsignal Gus to be applied by the signal adjustment module 130 so theultrasonic US signal is reduced by enough to avoid audio band distortionproducts.

In a more complex embodiment the signal adjustment module 130 mayrepresent an adjustable non-linearity, which is adapted, as describedabove.

As above, control block 120 b may also provide other control signals Custo the signal adjustment module 130 to adjust other parameters of theultrasonic component signal Cu, for example the duration or duty cycleof ultrasonic signal bursts.

In some devices, particularly where the transducer is driven from aboosted supply, the main audio path is subject to control by a speakerprotection module to avoid damage to the transducer. Such modules mayuse a speaker plant model that may derive operational variables relatingto the loudspeaker such as displacement of membrane, velocity ofmembrane, acceleration of membrane, or temperature of the voice coil.Any signal limiting to prevent excessive excursion may occur upstream ofthe signal Va and will tend to be at lower audio frequencies, where mostspeaker excursion components occur.

Thus FIG. 14 illustrates a further embodiment in which the ultrasonicsignal adjustment is controlled by operational speaker variables Pspsuch as displacement of membrane, velocity of membrane, acceleration ofmembrane, or temperature of the voice coil. These are derived by aspeaker plant model 410 from inputs Vsp, Isp, representing the voltageand current in the voice coil of the speaker.

Specifically, FIG. 14 illustrates an embodiment in which an adaptivescheme is applied to the ultrasonic path in order to modify theultrasonic signal in such a way that the down conversion terms areminimized. In this embodiment, speaker voltage and current Vsp and Ispare input to a speaker model module 410 which generates variables Psp.In a simple embodiment scheme the signal adjustment module 130 is a gainelement. The parameters Psp are processed in a control block 120 c toprovide a gain control signal Gus to be applied by the signal adjustmentmodule 130. Operation is similar to that of FIG. 13, except that one ormore of the above operational speaker variables are used to detectpossible distortion rather than an estimate of the acoustic outputitself. As above, control block 120 c may also provide other controlsignals Cus to the signal adjustment module 130 to adjust otherparameters of the ultrasonic component signal Cu, for example theduration or duty cycle of ultrasonic signal bursts.

This may also assist in avoiding other issues from driving ultrasonicand audio components at the same time, e.g. excessive heating. Thus insome devices, particularly where the transducer is driven from a boostedsupply, the main audio path is subject to control for speaker protectionto avoid damage to the transducer. This audio signal cannot be adjustedwithout potentially damaging the speaker. Also from a ‘HiFi’ perspectivewe do not want to modify the processing of the audio components intransmit path unless absolutely necessary. Thus it is advantageous tomodulate the ultrasonic signal components to regulate any thermal issuescaused by the ultrasonic pulses rather than modulate the audio componentin response to the power dissipated by the ultrasonic pulses.

In embodiments described so far the components Ca and Cu are physicallycombined by combiner 102 or similar, for example a simple signaladdition or summer, before being applied to the loudspeaker 20. In thiscase the summed signal might (after any required driver amplifier) beapplied to one terminal of a voice coil 21 of a loudspeaker 20 and theother terminal connected to ground as illustrated in FIG. 15, view (a).It should be understood that the electrical signal components Ca and Cumay be transferred to the loudspeaker or other electro-acoustictransducer in other ways to provide an acoustic output comprising therequired ultrasonic and audio components. For instance FIG. 15, view (b)illustrates the audio and ultrasonic components being applied separatelyto the two terminals of a voice coil 21. FIG. 15, view (c) illustratesthe summed signal being applied differentially across two terminals of avoice coil. FIG. 15, view (d) illustrates the audio and ultrasoniccomponents being applied to separate voice coils 21 a and 21 b, perhapsindividually optimized for the respective frequency ranges. Variationsof these connections will be apparent to those in the field. Other typesof transducer, for example electrostatic or piezo-electric may beemployed with equivalent connections. However in all cases there will bea common mechanical coupling to a structure such as a loudspeaker conethat will interface to the external air, and interaction between the twosignals may cause intermodulation of the acoustic components.

As used herein, the term ‘module’ shall be used to at least refer to afunctional unit or block of an apparatus or device. The functional unitor block may be implemented at least partly by dedicated hardwarecomponents such as custom defined circuitry and/or at least partly beimplemented by one or more software processors or appropriate coderunning on a suitable general purpose processor or the like. A modulemay itself comprise other modules or functional units.

This software code may be stored in the device as firmware on somenon-volatile memory (e.g., EEPROM) to preserve program data when thebattery becomes discharged or is removed for replacement.

It should be understood—especially by those having ordinary skill in theart with the benefit of this disclosure—that the various operationsdescribed herein, particularly in connection with the figures, may beimplemented by other circuitry or other hardware components. The orderin which each operation of a given method is performed may be changed,and various elements of the systems illustrated herein may be added,reordered, combined, omitted, modified, etc. It is intended that thisdisclosure embrace all such modifications and changes and, accordingly,the above description should be regarded in an illustrative rather thana restrictive sense.

Similarly, although this disclosure makes reference to specificembodiments, certain modifications and changes can be made to thoseembodiments without departing from the scope and coverage of thisdisclosure. Moreover, any benefits, advantages, or solutions to problemsthat are described herein with regard to specific embodiments are notintended to be construed as a critical, required, or essential featureor element.

Further embodiments likewise, with the benefit of this disclosure, willbe apparent to those having ordinary skill in the art, and suchembodiments should be deemed as being encompassed herein.

1.-22. (canceled)
 23. Driver circuitry, configurable to provide anoutput signal comprising an ultrasonic signal component and an audiosignal component, the driver circuitry comprising: an ultrasonic signalgenerator for generating the ultrasonic signal component; wherein atleast two parameters of the ultrasonic signal component are dependent onthe level of the audio signal component.
 24. Driver circuitry as claimedin claim 23, wherein one of the at least two parameters of theultrasonic signal component is inversely dependent on the level of theaudio signal component.
 25. Driver circuitry as claimed in claim 23,wherein one of the at least two parameters of the ultrasonic signalcomponent is an amplitude of the ultrasonic signal component.
 26. Drivercircuitry as claimed in claim 25, wherein another of the at least twoparameters of the ultrasonic signal component is a bandwidth, pulseduration, duty cycle or repetition rate of the ultrasonic signalcomponent.
 27. Driver circuitry as claimed in claim 25 wherein saidultrasonic signal generation module is configured to control theamplitude of said ultrasonic signal component by applying a controlledgain to said ultrasonic input signal.
 28. Driver circuitry as claimed inclaim 23, wherein said ultrasonic signal generator is configured togenerate an ultrasonic source signal and to generate the ultrasoniccomponent signal by altering an amplitude of the ultrasonic sourcesignal.
 29. Driver circuitry as claimed in claim 28, wherein theultrasonic signal generator is configured to generate the ultrasonicsource signal from a stored waveform.
 30. Driver circuitry as claimed inclaim 29, wherein the stored waveform comprises the waveform of a signalburst.
 31. Driver circuitry as claimed in claim 29, wherein theultrasonic signal generator is configured to generate the storedwaveform on re-initialization, and store it for subsequent use. 32.Driver circuitry as claimed in claim 29, wherein the ultrasonic signalgenerator is configured to generate the stored waveform based on arandom bit sequence.
 33. Driver circuitry as claimed in claim 23,wherein said ultrasonic signal generator is configured to generate theultrasonic component with a required amplitude, and with a requiredpulse duration, duty cycle, repetition frequency and frequency span orbandwidth.
 34. Driver circuitry as claimed in claim 33, wherein saidultrasonic signal generator is configured to generate the ultrasoniccomponent signal by selecting from a plurality of stored waveformscorresponding to a set of respective required characteristics. 35.Driver circuitry as claimed in claim 33, wherein said ultrasonic signalgenerator comprises a first waveform generator and a second waveformgenerator, and wherein the ultrasonic signal generator is configured togenerate the ultrasonic component as an output either of the firstwaveform generator or of the second waveform generator, based on thelevel of the audio signal component.
 36. Driver circuitry as claimed inclaim 35, wherein the first waveform generator is configured forgenerating a waveform of a first type and the second waveform generatoris configured for generating a waveform of a second type, wherein thesecond type of waveform is simpler than the first type, and wherein theultrasonic signal generator is configured to generate the ultrasoniccomponent as an output of the first waveform generator when the level ofsaid audio signal component is above a threshold level.
 37. Drivercircuitry as claimed in claim 36, wherein the waveform of the first typeis a wideband noise-like waveform.
 38. Driver circuitry as claimed inclaim 37, wherein the first waveform generator comprises: a digitalnoise generator, for generating a noise sequence of values; a band-passfilter, for receiving the noise sequence and limiting the bandwidththereof to an ultrasonic range; and a window for selecting a set ofsamples output from the band-pass filter, corresponding to a requiredpulse duration.
 39. Driver circuitry as claimed in claim 38, wherein thedigital noise generator comprises a pseudo-random bit sequencegenerator, for generating a random sequence of values with a flatwideband frequency spectrum.
 40. Driver circuitry, configurable toprovide an output signal comprising an ultrasonic signal component andan audio signal component, the driver circuitry comprising: anultrasonic signal generator for generating the ultrasonic signalcomponent; wherein a plurality of parameters of the ultrasonic signalcomponent are dependent on a parameter of the audio signal component.41. Driver circuitry for providing an output signal comprising anultrasonic signal component and an audio signal component, wherein thedriver circuitry is configured to: receive an audio source signalcomprising an audio band signal component; receive an ultrasonic sourcesignal; adjust the ultrasonic source signal based on the audio sourcesignal to generate the ultrasonic signal component of the output signal;and combine the audio source signal with the ultrasonic signal componentto generate the output signal, wherein adjusting the ultrasonic sourcesignal comprises varying two or more parameters of the ultrasonic sourcesignal based on the audio source signal.
 42. An electronic devicecomprising driver circuitry as claimed in claim 23, wherein theelectronic device is at least one of: a portable device; a batterypowered device; a communications device; a computing device; a mobiletelephone; a laptop, notebook or tablet computer; a personal mediaplayer; a gaming device; and a wearable device.